Those concerned about the relative safety of gadolinium-based contrast agents used during MRI scans may be interested in an August 2019 report by Massachusetts General Hospital about a manganese-based agent (Mn-PyC3A). It appears to provide comparable tumour contrast enhancement whilst being safer because the body clears it more quickly and thoroughly.
https://www.eurekalert.org/pub_releases/2019-08/mgh-nca081519.php